Page 283 - Anatomy and Physiology of Farm Animals, 8th Edition

268 / Anatomy and Physiology of Farm Animals

responsible for these general effects are exact number and location vary with the
species. Commonly, one pair is visible
not known, but the thyroid hormones bind
VetBooks.ir to an intracellular receptor that regulates outside the thyroid gland and is therefore
gene transcription in each of its target
glands. The second pair is often buried in
cells. The biological effects of thyroid hor­ designated the external parathyroid
mones are known as calorigenic action the substance of the thyroid, and these are
and are associated with an overall increase the internal parathyroid glands. The pig
in the metabolism and use of carbohy­ lacks grossly visible internal parathyroids.
drates and lipids, which is consistent with The two types of parathyroid cells are
the increased use of oxygen and heat chief cells and oxyphil cells (Fig. 13‐9).
production. Chief cells are small, usually dark cells that
Chronic exposure of some animals to are associated with the production of par-
cold is associated with an increase in athyroid hormone (PTH, parathormone).
TSH, T , and T . The increased calori- The less numerous oxyphil cells are larger,
4 3
genic effect of the thyroid hormones with granular cytoplasm and a small, dark
should work to maintain normal body nucleus. These cells have been described in
temperature in the cold environment. horses and cattle (and humans) but are not
The response to the cold appears to be a found in other domestic species. Their
result of an increase in TRH release by function is unknown, although the fact
the hypothalamus. The hypothalamus is that they appear later in life has suggested
known to be the reflex center for other that they may be senescent chief cells.
reflexes involved with the regulation of Parathyroid Hormone. PTH, a peptide
body temperature on a short‐term hormone, is the major controller of the
basis (e.g., peripheral vasodilation or level of blood calcium and phosphate. It
vasoconstriction). does this by stimulating release of calcium
Thyroid hormones are essential for nor­
mal growth and development during ges­ and phosphate from bone, decreasing
excretion of calcium and increasing
tation and in young animals. Two systems excretion of phosphate by the kidney,
of special importance are the skeletal and and promoting the formation of active
nervous systems. Animals with thyroid vitamin D by the kidney (discussed later).
hormone deficits (hypothyroidism) do not In bone, PTH acts on both osteocytes and
attain normal stature and have a variety of osteoclasts to promote the release of
central nervous system abnormalities. In
humans with severe deficits at birth, men­
tal development can remain impaired
throughout life even if replacement ther­
apy is initiated. Cretinism is the term for
the human condition caused by a congeni­
tal lack of thyroid hormone and is charac­
terized by arrested physical and mental
development and lowered metabolic rate.

Other Endocrine Glands

Parathyroid Glands

Figure 13-9. An equine parathyroid gland with
The parathyroid glands are small aggre­ two large oxyphil cells (arrows) among numerous
gates of endocrine tissue within or near the chief cells. Source: Dellmann and Eurell, 1998.
thyroid gland. Most domestic animals have Reproduced with permission of John Wiley &
two pairs of parathyroid glands, but the Sons, Inc.